Optical fibre switching components are fundamental to modern global information systems. Single-stage matrix switches operating independently of the optical bit-rate and modulation formats, capable of reconfigurably interconnecting N optical inputs to M optical outputs (where N and M are generally, but not necessarily the same number), are particularly attractive. Many switches for achieving the required switching are limited in functional size to less than 64×64, and/or suffer from relatively poor noise performance. One method which provides good noise performance and is potentially more scalable than other optical switch technologies is to use reconfigurable holograms as elements for deflecting optical beams between arrays of optical inputs and optical outputs.
A known holographic optical switch, otherwise known as an optical shuffle, is shown in FIG. 1.
In FIG. 1, an array of optical sources 1 and an array of optical receivers 7 are arranged as the inputs and outputs of a holographic switch. For many applications, the sources and receivers may comprise cleaved or end-polished fibres. In other applications, the inputs may be light emitting sources such as lasers or LEDs, and the outputs may be photo-detectors. Each input 1 may transmit a different digital or analog optical signal through the switch to one (or possibly several) of the outputs 7. Thus up to N different inputs may be simultaneously passing through the switch at any instant. Each input may consist of a single-wavelength modulated by data; a number of different data sources operating at different wavelengths (e.g. a wavelength-multiplexed system); or a continuum of wavelengths. Although the switch is shown in cross-section in FIG. 1, the input & output arrays 1, 7 are typically 2-dimensional arrays, and the holographic switch occupies a 3-dimensional volume.
To achieve switching, the input array 1 is arranged behind a first lens array 2. Each optical signal emitted by the input array enters free-space, where it is collimated by one of the lenses in first lens array 2. Each collimated beam then passes through a first hologram display device 3. The first hologram display device 3 displays a holographic pattern of phase and/or intensity and/or birefringence which has been designed to produce a specific deflection of the optical propagation directions of the beams incident upon the device. The hologram pattern may also be designed such that each optical beam experiences a different angle of deflection. The first hologram display device 3 may also have the effect of splitting an individual beam into several different angles or diffraction orders. One application for utilising this power splitting effect is to route an input port to more than one output port.
The deflected optical signals propagate in free-space across an interconnect region 4 until they reach a second hologram device 5. The hologram pattern at second hologram device 5 is designed in such a way to reverse the deflections introduced at the first hologram display device 3 so that the emerging signal beams are parallel with the system optic axis again.
The optical signals then pass through a second lens array 6 where each lens focuses its associated optical signal into the output ports of a receiver array 7. Thus the hologram pattern displayed on first hologram display device 3 and the associated “inverse” hologram pattern displayed on second hologram display device 5 determine which output fibre or fibres 7 receive optical data from which input fibre or fibres 1. The interconnect region 4 allows the signal beams to spatially reorder in a manner determined by the specific hologram patterns displayed on the first 3 and second 5 hologram display devices. The switch also operates reversibly such that outputs 7 may transmit optical signals back to the inputs 1.
The system shown in FIG. 1 (and functionally equivalent configurations utilising planes of symmetry within the switch optics) is well known as a method for static optical shuffle, using fixed hologram recordings as first 3 and second 5 hologram display devices whereby the input signals are “hard-wired” to specific outputs.
It has been proposed to extend the optical shuffle of FIG. 1 to provide a reconfigurable switch by displaying hologram patterns on a spatial light modulator (SLM). There are however a number of practical design problems associated with the migration from a static optical shuffle to a reconfigurable switch. Among these are the following:    1) Known SLMs, using a ferroelectric liquid crystal provide binary phase modulation and such phase modulation can be    2) polarisation-insensitive. However, the maximum theoretical diffraction efficiency for a binary phase device is only 40.5%. For example, the architecture shown in FIG. 1 uses two SLM devices, and hence the maximum net diffraction efficiency of this system is 16.4%. The diffraction efficiency of holographic system would be improved significantly by using multiple phase modulation. For many applications this multiple phase modulation must be polarisation-insensitive. It is desirable that the phase may be varied continuously between 0 and (at least) 2π.    3) In order to implement a holographic switch using two SLMs, an appropriate set of hologram patterns must be chosen. This hologram set must be capable of routing any input channel to any input channel whilst keeping the crosstalk figures within specified values. In particular, the hologram set must be optimised to prevent beams associated with unwanted diffraction orders from being launched down the wrong channel. Increasing the number of phase levels tends to result in a decrease in the strength of the unwanted diffraction orders.    4) A convenient method of constructing reconfigurable holograms for use within an N×N switch would be to integrate a layer of liquid crystal material above a silicon circuit. This type of SLM typically operates in reflection rather than transmission, and the switch layout shown in FIG. 1 is therefore no longer appropriate.
Accordingly the present invention aims to address at least some of these issues.